An operating valve is known in which a spool is disposed in a spool bore of a valve body for selectively establishing and blocking communication between a pump port, an actuator port and a tank port. In such operating valve, fluid leakage from the actuator port to the tank port is blocked by the fitted portion between the spool bore and the spool so that a holding pressurized fluid in the actuator may not flow to a tank. In this case, by reducing a gap in the fitted portion between the spool bore and the spool, leakage of the holding pressurized fluid in the actuator port may be prevented to a certain extent. However, since a smaller gap may cause greater sliding resistance of the spool, a greater operation force becomes necessary.
Therefore, in the prior art, a lock valve is provided in a line connected to the actuator or at another appropriate position. However, such arrangement inherently causes high cost.
On the other hand, in Japanese Unexamined Patent Publication (Kokai) No. Showa 62-147101, there is proposed an operating valve which is provided with a pressure compensation valve capable of preventing fluid leakage from the actuator port to the tank port.
However, in the operating valve having the pressure compensation valve, a cut-out in the spool is large to make it impossible to assuredly avoid leakage of the holding pressurized fluid in the actuator port to the tank.
Therefore, it is an object of the present invention to provide an operating valve assembly with a pressure compensation valve which can make a required operation force for a spool small and can assuredly prevent leakage of a holding pressurized fluid acting on an actuator port to a tank.